When this limestone was forming, the snail and bivalve shells buried in it dissolved away, leaving gaps. The solid shell shapes you see are casts of the insides of these shells.
Portland, south Dorset

Christmas & New Year Opening Times 

Sat 24 December | CLOSED
Sun 25 December | CLOSED
Mon 26 December | CLOSED
Tues 27 Dec | OPEN, Bank Holiday hours
Wed 28 Dec | OPEN, normal hours
Thurs 29 Dec | OPEN, normal hours
Fri 30 Dec | OPEN, normal hours
Sat 31 Dec | OPEN – 9am to 3pm (early closing)
Sun 1 Jan 2023 | CLOSED
Mon 2 Jan | OPEN, Bank Holiday hours
Tues 3 Jan | OPEN, normal hours